Comparison of control strategies for single-stage partial nitrification-anammox granular sludge reactor for mainstream sewage treatment—a model-based evaluation

The low ammonium concentration, low temperature, presence of organic matter, and large variation of influent ammonium load pose serious challenges for the application of PN/AMX (partial nitrification-anammox) reactor in the mainstream wastewater treatment. Previous mathematical simulation studies of PN/AMX granule reactor mainly concentrated on the steady-state modeling. The steady-state simulation cannot be used for developing control strategies under dynamic condition. In this study, four control strategies were evaluated on their abilities to minimize the impact of feed disturbances on autotrophic nitrogen removal in mainstream wastewater. The four control strategies included are the following: (A) direct airflow adjustment to maintain the fixed NH₄ ⁺ set point, (B) fixed NH₄ ⁺ set point control manipulated by DO concentration with DO limit, (C) constant DO control strategy, and (D) adaptive change of NH₄ ⁺ set point control based on the feed disturbance (NH₄ ⁺ set point value achieved by DO concentration manipulation with DO limit). The results indicated that the control strategy A successfully implemented for high NH₄ ⁺ strength wastewater treatment cannot be directly transferred into the mainstream wastewater treatment, in which high NO₂ ⁻ accumulation was resulted during the NH₄ ⁺ peaks at the low-temperature period. Satisfactory TN removal could be achieved by maintaining either fixed or variable bulk NH₄ ⁺ set point values calculated based on the feed disturbances (control strategies B and D). The DO limit imposed on the DO concentration manipulation to derive the desired NH₄ ⁺ set point values was essential for the successful implementation of control strategies B and D. The control strategy C with constant DO concentration was not feasible for the PN/AMX process under dynamic feed disturbances. The control simulation results and the control variable sensitivity analysis indicated that the NH₄ ⁺ concentration was a better control variable than the DO concentration.